1. Field of the Invention
The present invention relates to thermal insulators for rocket motors.
2. Discussion of Prior Art
Rocket motor designs of the type which are of interest today have existed since the late 1940's. During the intervening years. the design, development and engineering of motor hardware components have progressed appreciably contributing to the continually improving efficiency of associated missiles.
Two fundamental chemical parameters have imposed temperature limitations on motors. Firstly, the burning rates of propellants used in solid propellant rocket motors are related to the propellant temperature and in most instances the control of the burning rate, and consequently, control of the missile, becomes difficult above 60.degree.-80.degree. C., the exact temperature depending on the propellant in use. Secondly, conventional organic polymers, on which many of the adhesives and rubbers used in rocket motor hardware are based, typically start to lose strength at about 100.degree. C. and frequently commence to decompose near 200.degree. C. (Advanced polymers are slowly becoming available which, although of lower strength, may be expected to raise these temperatures but only by a limited amount, say to 200.degree. C. and 400.degree. C. respectively). With a few exceptions the above two fundamental parameters have not inhibited the design of rocket motors to date.
However, an era is approaching when aerodynamic heating (aeroheat) is likely to become increasingly severe. This is inevitable as the speeds of aircraft increase resulting in higher aeroheat temperatures due both to the carriage by those aircraft in captive flight of underslung missiles and to the increasing speed required in missile free flight in order both to attack other high speed aircraft and to avoid interception.
It is very unlikely that propellant technology will change significantly, hence it will be necessary to incorporate a thermal barrier to protect the propellant from aeroheat. This could be on the outer or the inner surface of the motor body and certainly in the former and probably in the latter instance will incur mass and volume penalties. It will also be necessary to modify or protect any organic polymer based hardware materials. If modified, a mass penalty due to their lower strength may be incurred and, if protected, a thermal barrier on the outer surface of the body, as for protection of the propellant, will be required.
Desirably, any protection added also complies with the current military requirements for insensitive munitions, i.e. munitions which are not operationally activated without control in the event for example of bump, shock, vibration or in a fuel fire in storage or under bullet or shrapnel attack. Furthermore such protection desirably has suitable mechanical properties in typical environmental conditions e.g. over a wide range of environmental temperatures and humidities and other atmospheric conditions e.g. wind, rain, hail etc.
It is therefore appropriate to provide as a thermal insulator to protect against aeroheat a material which is specially selected to provide an optimum combination of properties, whereby there is no substantially adverse effect on the cost, weight and volume or performance of rocket motor designs incorporating such a material.